(1) Field of the Invention
The present invention generally relates to an apparatus and method for etching a printed circuit board, and more particularly to an etching apparatus and method for etching a printed circuit board while the printed circuit board is horizontally transported in a transporting direction so as to keep a height of the printed circuit board at a transport position during etching.
(2) Description of the Prior Art
Generally, printed circuit boards are placed on various electronic devices. With development of high-performance, miniaturized electronic devices, there is the need for the printed circuit boards with high-density interconnections and with multi-layer printed patterns with thin wiring. On the other hand, digital signals, instead of analog signals, are becoming the dominant signals which are delivered through the printed circuits on the printed circuit boards. With the use of digital signals, the ratio of high-frequency circuits to the other printed circuits on the printed circuit boards are becoming greater, the high-frequency circuits processing the digital signals.
Therefore, in order to produce the multi-layer printed circuit boards with high-density interconnections, it is necessary that the manufacturing process steps are carried out with accurate widths of the interconnecting wires and with accurate distances between the adjacent wires on the printed circuit boards.
Generally, in order to produce a pattern of a film such as copper deposited over a board which forms a printed circuit on the board, it is necessary to carry out an etching process in which selected areas of the film deposited over the entire surface, as defined by a photoresist, are locally etched away to leave only the metal for interconnections. Thus, to realize high accuracy of the multi-layer printed circuit boards with high-density interconnections mentioned above, it is necessary to take into account the following items: 1) uniform the rate of etching a top surface of a printed circuit board and the rate of etching a bottom surface thereof; 2) uniform the rates of etching locally separate portions on the same surface of a printed circuit board; and 3) control the etching quantity in accordance with the thickness of a metal film deposited over the entire surface.
Horizontal-transport type etching machines are frequently used because the etching machines of this type can perform the etching of a printed circuit board with relatively accurate dimensions of the pattern of the film etched away. In the horizontal-transport type etching machine, a printed circuit board is horizontally transported so that a height of the printed circuit board during etching is kept at a transport position. The etching machine mentioned above includes an upper spray pipe above the transport position on which upper spray nozzles are arranged to inject an etchant into a top surface of the printed circuit board, and a lower spray pipe below the transport position on which lower spray nozzles are arranged to inject an etchant into a bottom surface of the printed circuit board.
The upper spray pipe and the lower spray pipe are connected to a spray pump, and the etchant under pressure is supplied by the spray pump to these spray pipes, so that the etchant is injected by the upper spray nozzles and the lower spray nozzles to the printed circuit board. Thus, selected areas of a metal film deposited over the entire surface of the printed circuit board, as defined by a photoresist, are locally etched away.
A conventional etching machine which is the horizontal-transport type described above comprises the upper spray pipe and the lower spray pipe, wherein the number of the upper spray nozzles on the upper spray pipe and the number of the lower spray nozzles on the lower spray pipe are the same, and the pressures of the etchant injected by each upper spray nozzle and of the etchant injected by each lower spray nozzle are the same. Further, in the conventional etching machine, a height of each of the upper spray nozzles above the transport position is substantially equal to a height of each of the lower spray nozzles below the transport position. In addition, the conventional etching machine comprises a single etching tank and a plurality of spray pipes within the etching tank, and all the pressures of the etchant injected by the individual spray nozzles are the same, and the pressures of the etchant injected cannot be controlled independently.
The conventional etching machine in which the spray nozzles are thus arranged has a problem in that although the etchant injected by each lower spray nozzle to the bottom surface of the printed circuit board will fall to the bottom of the etching tank, the etchant injected by each upper spray nozzle to the top surface of the printed circuit board will not immediately fall to the bottom of the etching tank but remains on the top surface.
As a certain amount of the etchant remains on the top surface of the printed circuit board, the remaining etchant will become an obstacle to etch the printed circuit board with a new etchant subsequently injected by the upper spray nozzles. The metal film at corresponding portions of the printed circuit board is not effectively etched away, and the etching ability will deteriorate and the etching speed will decrease. Thus, the efficiency of the etching will be lowered and it is difficult to perform the etching of the printed circuit board with accurate dimensions of the pattern of the film etched away.
A conceivable method to resolve the above problem is to increase the pressure of the etchant injected by each upper spray nozzle to a high pressure level enough to remove the remaining etchant on the top surface of the printed circuit board, allowing the top surface to be effectively etched with a new etchant subsequently injected by the upper spray nozzles. However, the increased pressure level of the etchant injected cannot exceed the strength of the photoresist. Thus, when the above method is applied, it is difficult to increase the pressure of the etchant injected by each upper spray nozzle to a sufficiently high pressure level at which the remaining etchant can be completely removed.
Another method to resolve the above problem is to decrease the pressure of the etchant injected by each lower spray nozzle relative to the pressure of the etchant injected by each upper spray nozzle, so as to uniform the rate of etching the top surface of a printed circuit board and the rate of etching the bottom surface. However, when this method is applied, the limitation of the decreased pressure level exists, and the etching speed of the entire etching machine will decrease and the efficiency of the etching will deteriorate.
When the etching of an inside layer of a multi-layer printed circuit board is performed by using a conventional etching machine in which the height of each of the upper spray nozzles above the transport position is substantially equal to the height of each of the lower spray nozzles below the transport position, the etching speed relating to the inner or middle areas on the top of the inside layer becomes lower than the etching speed relating to the outer or peripheral areas on the top of the inside layer. For this reason, lines of the pattern of the film in the inner areas on the top of the inside layer become thicker and lines of the pattern of the film in the outer areas on the top of the inside layer become thinner after the etching is performed, even if the thicknesses of photoresists in the inner areas and in the outer areas are the same.
Therefore, it is difficult for the conventional etching machine to realize the etching of the multi-layer printed circuit board with accurate dimensions of the film pattern which dimensions are in accordance with the line width of a desired circuit.
On the other hand, when the etching of an outside layer of a multi-layer printed circuit board is performed by using the conventional etching machine mentioned above, the lines of the film pattern in the outer areas on the outside layer become thicker and the lines of the film pattern in the inner areas thereon become thinner since the film on the outside layer is plated. For this reason, the etching speed relating to the inner areas on the outside layer is likely to be higher than the etching speed relating to the outer areas thereon. Even if the thicknesses of photoresists in the inner areas and in the outer areas are the same, the widths of the lines in the outer areas will become too great and the widths of the lines in the inner areas will become too small.
Therefore, it is difficult for the conventional etching machine to realize the etching of the multi-layer printed circuit board with accurate dimensions of the pattern of the film which dimensions are in accordance with the line width of a desired circuit.
Further, the conventional etching machine comprises a single etching tank and a plurality of spray pipes within the etching tank, as described above. All the pressures of the etchant injected by the individual spray nozzles are the same, and the pressures of the etchant injected cannot be controlled independently. Therefore, there are only two methods for controlling the etching quantity in accordance with the thickness of a metal film deposited over the entire surface with the conventional etching machine, that is: one is to control the pressure of the etchant injected by each spray nozzle; and the other is to control the transporting speed of a printed circuit board by the transport mechanism.
The conventional etching machine usually has a single etching tank, and the range of pressures of the etching injected by each spray nozzle which can be suitably controlled by the conventional etching machine is limited and narrow. For this reason, in order to change considerably the etching quantity, it is necessary to change considerably the transporting speed of the printed circuit board by the transport mechanism. However, when the changing of the transporting speed is applied, a change in the etching quantity responsive to a significantly small change in the transporting speed becomes excessively great. Thus, it is very difficult to control the etching quantity in accordance with the thickness of a metal film deposited over the entire surface by changing the pressure of the etchant injected to the printed circuit board or changing the transporting speed of the printed circuit board.